1. Field of the Invention
The present invention relates generally to slab reheat furnaces and, more particularly, to a technique for controlling the operation of such furnaces.
2. Description of the Prior Art
Elongated metal strips are produced in a facility known as a hot strip mill by rolling a slab, bar, or other bulk form of metal through successive stands of rollers. For convenience various bulk forms of metal will be referred to as slabs. As a slab progresses through the mill, it is increasingly thinned and elongated until it becomes a strip of thin metal. It will be appreciated that a great deal of work must be done by the rollers which compress the slab in order to work the slab into finished strip form. Because metal at an elevated temperature exhibits less resistance to deformation than metal at a lower temperature, the initial gross reductions in thickness are made while the slab is at an elevated temperature. Slabs are heated to a typical elevated temperature of about 2200 degrees Fahrenheit (.degree.F.) in a so-called slab reheat furnace. A slab reheat furnace thus performs the function of heating slabs from ambient temperature as stored in a "slab yard" to a desired elevated temperature deemed to be appropriate for the particular slab material and the type of rolling to follow.
On a production line a number of slabs are fed into the furnace sequentially. The speed of the slabs through the furnace and the temperature levels in the furnace are selected so that each slab being discharged from the furnace comes as close as possible to its desired temperature. Heating slabs to elevated temperatures would be relatively simple if each slab were of the same composition and dimensions, and were to be rolled to the same thickness in the same period of time. A typical hot strip mill does not operate in such a consistent manner, however, and slabs vary greatly in composition, dimension, and in processing requirements. Mill delays, whether planned or unplanned, also influence the travel of the slabs through the furnace. In short, depending upon individual requirements, slabs have to be heated to different temperatures and this means that the heating capabilities of the furnace must be adjusted as accurately as possible.
It has long been recognized that manual control of slab reheat furnaces is not the most efficient way to operate such furnaces. Slab composition, thickness, width, extraction interval, final desired thickness, and final desired temperature, as well as mill delays, all interact to make it exceedingly difficult to properly control the furnace to achieve the goal of heating each slab to a particular desired temperature. Under manual control, the differences among various slabs rarely are taken into account and slabs exiting the furnace often are heated improperly. Also, it is of increasing concern that much fuel may be wasted in the operation of a reheat furnace due to improper control.
In response to these concerns, the use of a process computer control for a slab reheat furnace was developed. The First Furnace Temperature Control Patent represented a significant step forward in utilizing computer technology to control the operation of a slab reheat furnace. The First Furnace Temperature Control Patent provides significant advantages in the operation of reheat furnaces, including:
1. Reduced fuel consumption. This benefit arises from the ability to control the slab heating at all times in accordance with strategy prescribed by efficient operation of the mill.
2. Increased furnace capacity. This is because the control system can respond quickly to changes in material flow and various delays. Slabs also can be delivered to the mill rolling equipment at a temperature which avoids the need for delays before entering the rolling equipment or for rolling at reduced speeds. In other words, the rest of the mill equipment can be operated at full capacity because slabs are delivered to the equipment at the proper temperature.
3. Better surface quality. Automatic control is sufficiently accurate that excessive scale formations, melting of the surface of the slab, and other undesirable characteristics are avoided.
Automatic reheat furnace control systems prior to the First Furnace Temperature Control Patent were not entirely satisfactory because they did not directly regulate slab temperature. They depended upon stored models to estimate slab temperature independently of specific information concerning the temperature of each slab as it progressed through the furnace.
The First Furnace Temperature Control Patent disclosed and claimed a greatly improved method and apparatus for controlling operation of a slab reheat furnace. In that patent, the average temperature of a slab in a given zone was predicted as a function of the gas temperatures in the zone, the thermal properties of the slab, the dimensions of the slab, the location of the slab within the zone, the rate of movement of the slab, and the thermal history of the slab. The predicted average temperature then was compared with a desired temperature at the same location based on a predetermined desired slab temperature trajectory. A performance index was established as a function of the combined comparisons for all slabs within the zone. The performance index was used to calculate a temperature setpoint, that is, a desired zone temperature. The heat output of the furnace was adjusted in accordance with the magnitude and direction of the difference between the setpoint and measured zone temperatures. The First Furnace Temperature Control Patent provided, for the first time, a truly automatic and effective technique for controlling the operation of a slab reheat furnace.
Although the First Furnace Temperature Control Patent represents a significant advance in the technology, certain concerns still have not been addressed. One of these concerns relates to the manner in which the performance index is determined. In the patent, the calculated average temperature of each slab was compared with a predetermined desired temperature of the slab at a given location. The temperature deviations of all slabs in a given zone were calculated periodically and the deviations were averaged. Because the temperature of slabs near the exit end of the zone is more critical than the temperature of slabs near the entrance to the zone, the temperature deviations of the slabs were weighted in favor of those near the exit end of the zone. The weighted temperature deviations than were summed to establish a zone performance index. This is a fairly complex technique to derive a performance index and, although the technique is accurate, it does not address itself to the thermal requirements of individual slabs.
Another concern not addressed by the First Furnace Temperature Control Patent relates to the means by which temperatures existing within the furnace are determined. In the patent, a thermocouple was placed on the roof of each zone of the furnace, as well as in the exhaust stack. A thermocouple was located at the transition point between a preheat zone and a heat zone within the furnace. In addition, radiation pyrometers were located at the transition from the preheat zone to the heat zone and from the heat zone to a soak zone to sense the temperature of each slab as the slab passed from one zone to the other. Obviously, if a number of thermocouples and pyrometers are employed in a furnace, a good indication of the heat distribution within the furnace can be determined. The greater the number of these sensors, the easier it will be to determine the heat profile within the furnace; in turn, the easier it will be to control operation of the furnace.
Even in the First Furnace Temperature Control Patent, however, the temperature in each zone was assumed to vary linearly from the sensed entry temperature to the sensed exit temperature. That was a simplistic assumption which did not accurately predict the radiation heat source temperature experienced by each slab. In many existing furnaces temperature sensors are placed at only one location along the length of each zone. Although it would be possible to modify existing furnaces to add more thermocouples and pyrometers, it would be desirable to employ existing equipment, if possible, to achieve proper furnace control. It also would be desirable for the assumed temperature distribution throughout the furnace to more accurately reflect the temperatures actually experienced by the slabs.